Methods and apparatus for synchronization of data frames

ABSTRACT

In accordance with an example embodiment of the present invention, a first bit sequence of a first length is assigned to a first group of signaling bits. Further, a second bit sequence of a second length is assigned to a second group of signaling bits. The first bit sequence is scrambled with a first scrambling sequence, and the second bit sequence is scrambled with a second scrambling sequence different from the first scrambling sequence. A first and a second orthogonal frequency-division multiplexing (OFDM) symbol are assigned to the first and the second scrambled bit sequences respectively, and the first and second orthogonal frequency-division multiplexing (OFDM) symbols are transmitted as synchronization symbols of a data frame. Further, a corresponding method for receiving the data frame, and apparatuses for transmission and reception are disclosed.

RELATED APPLICATION

This application was originally filed as PCT Application No.PCT/IB2010/053604 filed Aug. 10, 2010, which claims priority benefit toUnited States Provisional Patent Application No. 61/236,604, filed Aug.25, 2009.

TECHNICAL FIELD

The present application relates generally to synchronization of dataframes, for example data frames of a broadcasting system.

BACKGROUND

Many transmission systems, for example broadcasting transmissionsystems, are built according to a public or proprietary standard. Thus,it can be made sure that transmitters and receivers of the transmissionsystem may work together. However, standards evolve, and in order tosecure that devices that are built according to a newer version of astandard may communicate with devices that are built according to anearlier version of the standard, the standard has to be designedbackward compatible, and the devices have to be built accordingly. Forexample, after the introduction of color television (TV),black-and-white TV sets were still able to decode a color TV signal anddisplay a black-and-white image.

Also digital TV standards evolve, for example standards of the digitalvideo broadcasting (DVB) family of standards, such as digital videobroadcasting satellite (DVB-S), digital video broadcasting cable(DVB-C), digital video broadcasting terrestrial (DVB-T), digital videobroadcasting handheld (DVB-H), and/or the like. For example, DVB-T hasevolved into second generation DVB-T2. Even though backwardcompatibility was not a goal for DVB-T2, compatibility requirements thatDVB-T receivers do not get confused by a DVB-T2 signal.

Further, digital video broadcasting next generation handheld (DVB-NGH)is based in part on the DVB-T2 system. As with DVB-H, adaptations aremade in order to provide for requirements of a battery powered receiver,for example a DVB-NGH receiver in a handheld device like a mobilecomputer, a mobile telephone, and/or the like. Again, a compatibilityrequirement is that a DVB-T2 receiver does not get confused by a DVB-NGHtransmitter. For this purpose, a data field in a frame or packet may bedefined to have only a limited number of possible values for use inDVB-T2. For example, a 4-bit field may have defined only the values“0000”, “0001”, and “0010” in DVB-T2. Other values (for example “0100”)may be marked “for future use”. These other values may then be used in aDVB-NGH system or any other system evolving from DVB-T2. A DVB-T2receiver may ignore these other values.

SUMMARY

Various aspects of examples of the invention are set out in the claims.

According to a first aspect of the present invention, a first bitsequence of a first length is assigned to a first group of signalingbits. Further, a second bit sequence of a second length is assigned to asecond group of signaling bits. The first bit sequence is scrambled witha first scrambling sequence, and the second bit sequence is scrambledwith a second scrambling sequence different from the first scramblingsequence. A first and a second orthogonal frequency-divisionmultiplexing (OFDM) symbol are assigned to the first and the secondscrambled bit sequences, and the first and second orthogonalfrequency-division multiplexing (OFDM) symbols are transmitted assynchronization symbols of a data frame.

According to a second aspect of the present invention, an orthogonalfrequency-division multiplexing (OFDM) signal comprising a first and asecond orthogonal frequency-division multiplexing (OFDM) symbol isreceived. The first and second orthogonal frequency divisionmultiplexing (OFDM) symbols are used for synchronization of a dataframe, wherein the first orthogonal frequency-division multiplexing(OFDM) symbol corresponds to a first scrambled bit sequence and thesecond orthogonal frequency-division multiplexing (OFDM) symbolcorresponds to a second scrambled bit sequence. The first scrambled bitsequence is descrambled with a first scrambling sequence and the secondscrambled bit sequence with a second scrambling sequence different fromthe first scrambling sequence. A first group of signaling bits isdecoded from the first descrambled bit sequence and a second group ofsignaling bits is decoded from the second descrambled bit sequence.

According to a third aspect of the present invention, an apparatus isdescribed comprising a processor configured to assign a first bitsequence of a first length to a first group of signaling bits, and toassign a second bit sequence of a second length to a second group ofsignaling bits. The processor is further configured to scramble thefirst bit sequence with a first scrambling sequence, and to scramble thesecond bit sequence with a second scrambling sequence different from thefirst scrambling sequence. The processor is also configured to assign afirst and a second orthogonal frequency-division multiplexing (OFDM)symbol to the first and the second scrambled bit sequences, and totransmit the first and second orthogonal frequency-division multiplexing(OFDM) symbol as synchronization symbols of a data frame.

According to a fourth aspect of the present invention, an apparatus isdescribed comprising a processor configured to receive an orthogonalfrequency-division multiplexing (OFDM) signal comprising a first and asecond orthogonal frequency-division multiplexing (OFDM) symbol. Theprocessor is further configured to use the first and second orthogonalfrequency division multiplexing (OFDM) symbol for synchronization of adata frame, wherein the first orthogonal frequency-division multiplexing(OFDM) symbol corresponds to a first scrambled bit sequence and thesecond orthogonal frequency-division multiplexing (OFDM) symbolcorresponds to a second scrambled bit sequence. The processor is alsoconfigured to descramble the first scrambled bit sequence with a firstscrambling sequence and the second scrambled bit sequence with a secondscrambling sequence different from the first scrambling sequence. Theprocessor is further configured to decode a first group of signalingbits from the first descrambled bit sequence and a second group ofsignaling bits from the second descrambled bit sequence.

The apparatuses may comprise at least one memory that containsexecutable instructions that if executed by the processor cause theapparatus to perform the above described functions.

According to a fifth aspect of the present invention, a computer programproduct comprising computer program code for use with a computer, acomputer readable medium, and a computer program are disclosed forperforming the above described functions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the presentinvention, reference is now made to the following descriptions taken inconnection with the accompanying drawings in which:

FIG. 1 shows a transmission system according to an example embodiment ofthe invention;

FIG. 2 shows an example embodiment of a data frame comprisingsynchronization and signaling information;

FIG. 3 shows an example embodiment of a time domain structure of a P1symbol;

FIG. 4 shows an example embodiment of a frequency domain structure of aP1 symbol;

FIG. 5 shows an example embodiment of a data frame having multiple P1symbols and one or more P2 symbols;

FIG. 6 shows an example embodiment of a data frame having multiple P1symbols and no P2 symbol;

FIG. 7 shows an example embodiment of functional blocks of a P1 symbolgenerator;

FIG. 8 shows an example embodiment of an apparatus configured totransmit frames or packets of a data stream;

FIG. 9 is a flow diagram of a method for transmitting P1 symbols assynchronization symbols according to an example embodiment of theinvention;

FIG. 10 is a flow diagram of a method for receiving a frame or packetcomprising P1 symbols as synchronization symbols according to an exampleembodiment of the invention;

FIG. 11 shows an example embodiment of a timing synchronization unit fora receiver for the synchronization of up to four orthogonal frequencydivision multiplexing (OFDM) symbols; and

FIG. 12 shows an example embodiment of an apparatus configured toreceive a media stream.

DETAILED DESCRIPTION OF THE DRAWINGS

An example embodiment of the present invention and its potentialadvantages are understood by referring to FIGS. 1 through 12 of thedrawings.

FIG. 1 shows a transmission system 100 according to an exampleembodiment of the invention. A service provider 102 provides a servicefor transmission, for example a media stream comprising audio data,video data, a TV program, a data file, and/or the like, for broadcastingby transmitter 104. Transmitter 104 may comprise electronics to assembletransmission packets, apply signal processing and modulation to thesignals or transmission packets, provide a radio frequency (RF) signalfor transmission, and/or the like. Transmitter 104 may also comprise anantenna for transmitting the RF signal. Transmitter 104 provides abroadcast transmission signal 124 for an apparatus comprising astationary receiver 114, for example a TV set or a multimedia equipmentin a home, through a broadcast transmission 124. The broadcasttransmission signal 124 may be transmitted according to the DVB-T2standard or any other standard for digital terrestrial transmission.Transmitter 104 may also provide a transmission signal 126 for anapparatus comprising a mobile receiver 116, for example a mobile TV set,a personal digital assistant (PDA) comprising a broadcasting receiver, amobile phone comprising a broadcasting receiver, and/or the like. Thetransmission signal 126 for the mobile receiver 116 may be transmittedaccording to the DVB-NGH standard or any other standard suitable formobile transmission.

The standard for terrestrial transmission and the standard for mobiletransmission may have one or more commonalties in order to ease thetransmitter and receiver design. Thus, transmitters and receivers may bebuilt that are suitable for more than one standard without adding a highcost overhead.

FIG. 2 shows an example embodiment of a data frame 200 comprisingsynchronization and signaling information, for example a data frame inaccordance with a DVB system such as DVB-T2. Data frame or packet 200may be part of a DVB-T2 transmission stream of frames or packets, forexample a transmission stream carrying one or more services like mediastreams, video streams, audio streams, data files, and/or the like.DVB-T2 uses orthogonal frequency-division multiplexing (OFDM) fortransmission. A DVB-T2 frame or packet starts with a preamble comprisinga “P1” symbol 202 and one or more “P2” symbols 212.

The P1 symbol 202 has been designed with a number of properties thatmake it suitable for synchronizing the DVB-T2 frame at a receiver.Constraints for a DVB-T2 system may be that DVB-T2 reception is in manycases static and that a relatively large antenna may be used, forexample a rooftop antenna. The P1 symbol 202 may also carry lower layersignaling information, for example an indication of a transmission typeand basic transmission parameters of the data frame.

The P1 symbol 202 is followed by at least one P2 symbol 212. In theexample embodiment of FIG. 2, the P1 symbol 202 is followed immediatelyby at least one P2 symbol 212. The P2 symbol 212 comprises additionallower layer signaling information. In an example embodiment, signalinginformation P2 may comprise parameters that may be used to access datacarried in the data frame. In an example embodiment, signalinginformation of the at least one P2 symbol 212 comprises L1 pre-signalinginformation 222 and L1 post-signaling information 224. L1 pre-signalinginformation 222 enables the reception of the L1 post-signalinginformation 224 and may be used to enhance the efficiency of the coding.L1 post-signaling information 224 comprises parameters for accessingdata carried in the data frame. In an example embodiment, L1post-signaling information 224 comprises a configurable part 242 and adynamic part 244. An optional extension field 246 may further follow thedynamic part 244. Further a Cyclic Redundancy Code (CRC) field 248 maybe added. Unused parts of the P2 signaling information may be stuffed bypadding 250.

Other data may be transmitted in frame 200 beginning at symbol 214, forexample data of a service, for example a media stream comprising a TVprogram.

A transmission signal for a mobile apparatus, for example transmissionsignal 126 for mobile receiver 116 of FIG. 1, may also carry one or moreservices like media streams, video streams, audio streams, data files,and/or the like. The transmission signal for the mobile apparatus mayalso comprise P1 and P2 symbols as shown in FIG. 2.

Reception condition for a handheld device, for example a mobilereceiver, may be more difficult because of a small antenna, badreception conditions and/or the like, for example due to an indoorposition of the antenna, varying reception conditions due to movement ofthe device, and/or the like. A mobile system for reception of abroadcasting signal may therefore have higher requirements with respectto the robustness of the broadcasting signal. Therefore, a requirementof the mobile system may be that the synchronization properties of thebroadcasting signal are more robust.

FIG. 3 shows an example embodiment of a time domain structure of a P1symbol, for example P1 symbol 202 from FIG. 2, along a time axis 300. Inthe time domain, at least one part of the symbol may be repeated inorder to provide correlation properties for synchronization. Forexample, a first part A₁ of a main part A may be repeated as part Bbefore the main part A, and/or a second part A₂ may be repeated as partC after the main part A. In an example embodiment, the P1 symbol 202 inFIG. 3 comprises a main part A 302 that carries 1024 samples and extendsover a time T_(A)=112 μs. Main part A 302 may be divided into aplurality of parts, for example, a part A₁ 304 of a time T_(A1)≈59 μsand a part A₂ 306 of a time T_(A2)≈53 μs. Further, the P1 symbolcomprises a part B 314 of a time T_(B)≈59 μs and a part C 316 of a timeT_(C)≈53 μs. Part B 314 carries a frequency shifted copy of the part A₁.Part C 316 carries a frequency shifted copy of the part A₂. Thus,correlation properties of parts A₁ and B and/or A₂ and C, respectively,may be used in a receiver of the signal carrying symbol P1 202 toachieve time synchronization. By decoding the content of the P1 symbol,a service discovery may be initiated.

FIG. 4 shows an example embodiment of a frequency domain structure of aP1 symbol along a frequency axis 400. A pattern of active and inactivecarriers may be used to provide synchronization and/or detection of awanted signal, for example a DVB signal. Further, a sub-range may beused to provide further synchronization properties. In the exampleembodiment of FIG. 4, out of 853 carriers (numbered 0 . . . 852) ofrange 402 a number of 384 active carriers are used. Unused carriers areshown by short arrows 406, while active carriers are shown with longarrows 408. Active carriers may only span a sub-range 404 of all 853carriers. In an example embodiment, a sub-range 404 of 6.83 MHz is usedfor active carriers (carriers 44-809), while the range 402 of the 853carriers may span 7.61 MHz. The active carriers may be randomly orpseudo-randomly distributed in the sub-range. The random orpseudo-random distribution may be used by a receiver to achievefrequency synchronization and/or to detect the position of the P1 symbolin the transmission stream.

In an example embodiment, for example in a DVB-T2 system, the P1 symbolmay carry a group of 7 bits, for example bits carrying signalinginformation. The group of 7 signaling bits may be split into a firstfield S1 comprising 3 signaling bits and a second field S2 comprising 4signaling bits. According to an embodiment of the invention the S1 fieldmay have the values shown in Table 1:

TABLE 1 S1 field Preamble Format/ S1 P2 Type Description 000 T2_SISO T2preamble: The P2 part is transmitted in SISO format 001 T2_MISO T2preamble: The P2 part is transmitted in MISO format 010 NGH_SISO NGHpreamble: The P2 part is transmitted in SISO format 011 NGH_MISO NGHpreamble: The P2 part is transmitted in MISO format 100 reserved Thesecombination may be 101 reserved used for future systems 110 reserved 111reserved

The values “000” and “001” are defined for the DVB-T2 system. A value“000” of the S1 signaling bits indicates a single input-single output(SISO) system with one transmitting and one receiving antenna. A value“001” indicates a multiple input-single output (MISO) system withmultiple transmitting antennas and one receiving antenna.

The values “010” and “011” may be defined for a next generation handheld(NGH) system. A value “010” of the S1 signaling bits may indicate a SISOsystem, a value “011” may indicate a MISO system. Values “100”-“111” maybe reserved for future use. Similarly, one or more of these fields maybe used to indicate a multiple input-multiple output (MIMO) system.

By using values “000” and “001” for the T2_SISO and T2_MISO systems,respectively, as in the DVB-T2 system, backward compatibility with theDVB-T2 system may be achieved. A DVB-T2 receiver may decode the values“000” or “001” in the field and therefore received the remainder of theframe or packet. However, if a DVB-T2 receiver decodes the values “010”or “011” of the S1 signaling bits, it will not receive and/or decode theremainder of the frame or packet, as the DVB-T2 receiver may not becapable of decoding the frame or packet correctly. The DVB-T2 receivermay therefore ignore the complete frame or packet.

A DVB-NGH receiver may decode the values “010” or “011” and thereforereceive the whole frame or packet.

The S2 field may or may not be altered from the DVB-T2 standard. TheDVB-T2 standard defines the fast fourier transformation (FFT) size asshown in TABLE 2 and defines whether preambles in the transmission areof the same type (value “XXX0”) or whether different types of preamblesare used (value “XXX1”): TABLE 3.

TABLE 2 S2 field (DVB-T2, S1 = 00X) S1 S2 FFT size Description 00X 000XFFT size: 2K Indicates the FFT size of the 00X 001X FFT size: 8K symbolin the T2 frame 00X 010X FFT size: 4K 00X 011X FFT size: 1K 00X 100X FFTsize: 16K 00X 101X FFT size: 32K 00X 110X reserved These combination maybe used 00X 111X reserved for future systems

TABLE 3 S2 field (“mixed”-bit) S1 S2 Meaning Description XXX XXX0 Notmixed All preambles in the current transmission are of the same type asthis preamble XXX XXX1 Mixed Preambles of different types aretransmitted including at least one T2 preamble (S1 field = 00X) in everysuper-frame

Transmission parameters, for example modulation of the P1 symbol,transmission frequency, and/or the like may be identical for a DVB-NGHtransmission and a DVB-T2 transmission. Thus, a receiver may alreadydecode from the S1 field whether the frame or packet is part of a DVB-T2transmission or part of a DVB-NGH transmission. Therefore, in a DVB-NGHsystem, the S2 field may be used for signaling as described for theDVB-T2 system, or different signaling may be carried by the 4 signalingbits of the S2 field.

In an example embodiment, a bit sequence is assigned to the group ofsignaling bits, for example a bit sequence of a length corresponding tothe number of active carriers used for the transmission of the P1symbol. The bit sequence may have 384 bits in correspondence with thefrequency domain structure of the P1 symbol shown in FIG. 4.

The bit sequence may be a concatenated bit sequence. For example, 8different bit sequence values may be used for a first partial bitsequence SEQ_(S1) for the different values of the 3 signaling bits ofthe S1 field, and 16 different bit sequence values may be used for asecond partial bit sequence SEQ_(S2) for the 4 signaling bits of the S2field. The first partial bit sequence SEQ_(S1) may have a length of 64bits, and the second partial bit sequence SEQ_(S2) may have a length of256 bits. The concatenated bit sequence may concatenate the first andsecond partial bit sequence in the order SEQ_(S1)-SEQ_(S2)-SEQ_(S1).Thus, a total sequence length of 384 bits is achieved (256 bits+2*64bits).

The bit sequence may be scrambled by a scrambling sequence of the samelength. In an example embodiment, scrambling is performed by anXOR-function or by any other suitable function for reducing apeak-to-average power ratio (PAPR) of the signal.

In an example embodiment, the scrambling sequence is generated by apseudo-random generator, for example a pseudo-random binary sequence(PRBS) generator. The PRBS generator may be related to a polynomial ofdegree n (1+a₁*x+a₂*x²+ . . . a_(n-1)*x^(n-1)+x^(n); a₁, a₂ . . .a_(n-1)ϵ[0; 1]), for example a polynomial 1+x¹⁴+x¹⁵. Using a primitivepolynomial may provide a maximum length pseudo-random binary sequence.The repetition length of the pseudo-random binary sequence may becalculated from the degree n of the primitive polynomial as 2^(n)−1. Inthe case for the primitive polynomial 1+x¹⁴+x¹⁵ the repetition length ofthe pseudo-random binary sequence is 32767. Thus, after 32767 bits, thepseudo-random binary sequence repeats itself. In an example embodiment,the scrambling sequence for scrambling the bit sequence is a contiguouspart of the pseudo-random binary sequence, for example a contiguous partof 384 bits for scrambling a bit sequence of a length of 384 bits.

In an example embodiment, an orthogonal frequency-division multiplexing(OFDM) symbol is assigned to the scrambled bit sequence. The OFDM symbolmay use active carriers as described in relation to FIG. 4 and may havea time-domain structure as shown in FIG. 3. The OFDM symbol is thentransmitted as a synchronization symbol to a receiver, for example fromtransmitter 104 to receiver 114 or receiver 116 of FIG. 1. Thesynchronization symbol may be part of a data frame, a data packet, adata burst and/or the like.

FIG. 5 shows an example embodiment of a data frame having multiple P1symbols and one or more P2 symbols. A data frame 542 may comprise apreamble 540 and a part comprising user data starting at symbol 514. Thepreamble comprises at least a first P1 symbol 502 and a second P1 symbol504 and one or more P2 symbols 512. In an example embodiment, thepreamble may comprise further P1 symbols following the second P1 symbol504, for example a third P1 symbol 506 and a fourth P1 symbol 508.

The first P1 symbol 502 may correspond to a first group of signalingbits. The first group of signaling bits may comprise an S1 field and anS2 field as described in TABLES 1 to 3.

The first P1 symbol 502 is followed by a second P1 symbol 504. Thesecond P1 symbol 504 may correspond to a second group of signaling bits.In an example embodiment, the second group of signaling bits is a copyor a repetition of the first group of signaling bits comprising the S1and S2 fields.

The first and second group of signaling bits may be assigned a first andsecond bit sequence. In an example embodiment, the first group ofsignaling bits is assigned a first bit sequence of a first length, andthe second group of signaling bits is assigned a second bit sequence ofa second length. The first and second group of signaling bits maycomprise 7 signaling bits each, and the first and second bit sequencemay have a length of 384 bits each. In an example embodiment, the firstand second group of signaling bits are identical. Thus, also the firstand second bit sequences may be identical.

In an example embodiment, the first and second bit sequences arescrambled with different scrambling sequences. The first bit sequencemay be scrambled with a first scrambling sequence, for example of 384bits length, and the second bit sequence may be scrambled with a secondscrambling sequence of 384 bits length. In an example embodiment, thefirst and second scrambling sequences may be parts of a pseudo-randombinary sequence (PRBS) with a repetition length that is longer than eachscrambling sequence. For example, the first scrambling sequence may be afirst part of a PRBS of length 32767 bits, and the second scramblingsequence may be a second part of the same PRBS. The second part of thePRBS may be a part directly following the first part of the PRBS. Thus,a continuous output of a PRBS generator may be used to scramble thefirst and second bit sequences.

A first and second OFDM symbol is assigned to the first and secondscrambled bit sequences, respectively. The first and second OFDM symbolmay use the same set of 384 active carriers as described in relation toFIG. 4. By using different scrambling sequences for the first and secondbit sequence, the first and second OFDM symbols may be different.

Having more than one OFDM symbol for synchronization may improve thesynchronization properties of the transmission frame or packet. Usingdifferent OFDM symbols may make the preamble of the frame or packet morerobust to inter-symbol interference, for example in single-frequencynetworks with multipath propagation.

Using the a first group of signaling bits as described in relation toTABLES 1 to 3, and further assigning a bit sequence and a scramblingsequence to the first group of signaling bits as in the DVB-T2 systemmay provide backward compatibility for a DVB-T2 receiver. A DVB-T2receiver may be capable of decoding the first P1 symbol. It may findthat it is capable of decoding the frame or packet (S1=“000” or “001”),or it may discard the remainder of the frame or packet (S1=“010” or“011” or any other value).

A DVB-NGH receiver may also decode the first symbol. When it decodes theS1 field, it may find that the content of the S1 field is “010” or“011”. It may then conclude that the next symbol is a second P1 symbolthat may be used for signaling. It may further conclude that a secondscrambling sequence different from the first scrambling sequence isrequired for decoding the second P1 symbol, for example a continuationof the first scrambling sequence used for the first P1 symbol.

In an example embodiment, the second group of signaling bits may bedifferent from the first group of signaling bits. Thus, more informationmay be carried in the first and second P1 symbols. For example, thesecond group of signaling bits may carry signaling information of one ormore DVB-T2 P2 symbols. Thus, information is shifted from P2 symbols ofthe DVB-T2 system to P1 symbols in the DVB-NGH system. In an alternativeembodiment, the second group of signaling bits comprises informationdifferent from the information carried in the P2 symbols of the DVB-T2system.

In an example embodiment, the second P1 symbol 504 is followed by one ormore further P1 symbols, for example by a third P1 symbol 506. A fourthP1 symbol 508 may follow the third P1 symbol. The third P1 symbol 506and the fourth P1 symbol 508 may carry a third and a fourth group ofsignaling bits, respectively. The third and fourth group of signalingbits may be identical to the first group of signaling bits carrying theS1 and S2 fields. In an alternative embodiment, the third and fourthgroup of signaling bits may carry information different from the firstand/or second group of signaling bits. For example, the third and fourthgroup of signaling bits may carry signaling information of one or moreDVB-T2 P2 symbols, or they may carry different information.

A third bit sequence of a third length may be assigned to the thirdgroup of signaling bits, and a fourth bit sequence of a fourth lengthmay be assigned to the fourth group of signaling bits. In an exampleembodiment, the third and fourth group of signaling bits comprise 7signaling bits each, and the third and fourth bit sequence may have alength of 384 bits each.

In an example embodiment, the third and fourth bit sequences arescrambled with different scrambling sequences. The third bit sequencemay be scrambled with a third scrambling sequence, for example of 384bits length, and the fourth bit sequence may be scrambled with a fourthscrambling sequence of 384 bits length. In an example embodiment, thethird and fourth scrambling sequences may be parts of a pseudo-randombinary sequence (PRBS) with a repetition length that is longer than eachof the first, second, third and fourth scrambling sequences. Forexample, the third scrambling sequence may be a third part of a PRBS oflength 32767 bits, and the fourth scrambling sequence may be a fourthpart of the same PRBS. The third part of the PRBS may be a part directlyfollowing the second part of the PRBS, and the fourth part of the PRBSmay be a part directly following the third part of the PRBS. Thus, acontinuous output of a PRBS generator may be used to scramble the firstto fourth bit sequences.

A third and fourth OFDM symbol is assigned to the third and fourthscrambled bit sequences, respectively. The third and fourth OFDM symbolmay use the same set of 384 active carriers as described in relation toFIG. 4. By using different scrambling sequences for the third and fourthbit sequence, the third and fourth OFDM symbols may be different.

Having more than one OFDM symbol for synchronization may improve thesynchronization properties of the transmission frame or packet. Usingdifferent OFDM symbols may make the preamble of the frame or packet morerobust to inter-symbol interference.

The lengths of the second, third and fourth bit sequences may bedifferent from the length of the first bit sequence and from each other.The second, third and fourth scrambling sequences may have the samelengths as the second, third and fourth bit sequences, respectively. Thefirst to fourth scrambling sequences may be parts of a pseudo-randombinary sequence (PRBS) with a repetition length that is longer than eachof the first, second, third and fourth scrambling sequences. In anexample embodiment, the first to fourth scrambling sequences arecontinuous parts of a PRBS based on the primitive polynomial 1+x¹⁴+x¹⁵.

Each scrambled bit sequence is assigned an OFDM symbol. The lengths ofthe first to fourth scrambled bit sequences may correspond to the numberof active carriers of the first to fourth OFDM symbol, respectively, forexample the carriers of an OFDM symbol shown in FIG. 4. Thus, there is adirect mapping of each bit of the bit sequence to an active carrier ofthe respective OFDM symbol. For example, a scrambled bit sequence of 200bits may be assigned to 200 active carriers out of the 853 carriers ofthe OFDM symbol, or a bit sequence of 680 bits may assigned to 680active carriers out of the 853 carriers of the OFDM symbol. In anotherexample embodiment, higher order modulation may be applied on eachcarrier, and thus multiple bits may be assigned on each carrier.

By using different carriers for each P1 symbol, the synchronizationproperties may be adapted, for example according to present or futurereceiver capabilities, transmission channel conditions, and/or the like.

In an example embodiment, the first to fourth group of signaling bitscomprise 7 signaling bits each. Thus, a total number of 28 signalingbits may be transmitted in the 4 P1 symbols 502, 504, 506 and 508. Thesignaling bits may be used to announce parameters of the next and/orhigher level signaling, for example of the L1 pre-signaling 222, whichis so far transmitted in the one or more P2 symbols 212 of FIG. 2.Parameters transmitted in the P1 symbols may include an FFT size, aguard interval length, a pilot pattern, a frame length, and/or the like.

FIG. 6 shows an example embodiment of a data frame having multiple P1symbols and no P2 symbol. Frame or packet 642 comprises a preamble 640comprising four P1 symbols 602, 604, 606 and 608. The frame or packet642 also comprises a data part beginning at symbol 614. After the end offrame or packet 642, another frame or packet is transmitted beginningwith P1 symbols 622, 624, 626 and 628 followed by a data part startingat symbol 634.

Signaling data that is defined by the DVB-T2 standard to be transmittedin one or more P2 symbols may be transmitted at least in part in themultiple P1 symbols 602, 604, 606 and 608 and 622, 624, 626 and 628.Parts of the signaling data of the one or more P2 symbol may also betransmitted in the data part beginning at symbols 614 and 634, ordistributed over the data frame. Thus, a P2 symbol may not be needed.

FIG. 7 shows an example embodiment of functional blocks of a P1 symbolgenerator 700. Signaling bits, for example 51 and S2 signaling bits,and/or other signaling bits like L1 pre- and post-signaling bits, arereceived at inputs 702 and 704 and directed to a bit sequence mapping abit sequence mapping 706. For example, 7 signaling bits are received atthe bit sequence mapping 706 and mapped to a bit sequence, for exampleof a length of 384 bit. In an example embodiment, the bit sequence ismapped to a digital binary phase shift keying (DBPSK) at a DBPSK mappingblock 708. For example, a sequence generated by DBPSK mapping block 708may start with a value “+1”, which is not assigned to any carrier later.For a bit input of “0”, the last value of the DBPSK is repeated. For abit input of “1”, the last value of the DBPSK is inverted. Thus, if theprevious value was “+1”, the next value is “+1” for an input “0”, andthe next value is “−1” for an input “1”. The relation is shown in TABLE4:

TABLE 4 DBPSK mapping Previous value Input Next value Comment +1 0 +1Repeat previous value −1 0 −1 Repeat previous value +1 1 −1 Invertprevious value −1 1 +1 Invert previous value

At a scrambler block 710, the sequence is scrambled with a scramblingsequence. In an example embodiment, the scrambling sequence may comprisevalues “+1” and “−1”. The scrambling sequence may be generated by a PRBSgenerator. A “0”-value of the output of the PRBS generator is mapped to“+1”, a “1”-value is mapped to “−1”. The scrambler block 710 thenmultiplies a bit of the input sequence with a bit of the scramblingsequence.

At a carrier mappint 712, the scrambled bit sequence is mapped tocarriers of an OFDM symbol. A carrier map 714 is provided. The OFDMsymbol is then inverse fourier transformed at an inverse-fast-fouriertransform (IFFT) block 716 to generate a time representation of thesymbol. The fourier block size may be 1024 (1K). At a C-A-B structureblock 718, parts A1 and A2 of the symbol are repeated as parts B and Cto generate a C-A-B structure, as described in relation to FIG. 3. TheP1 symbol is provided at an output 720 for modulation in the assignedfrequency band.

For a first group of signaling bits, the PRBS generator of scramblerblock 710 is initialized with a fixed value, for example the value“100111001000110”. After generation of a scrambling sequence of a lengthcorresponding to a first length of a first bit sequence, the PRBSgenerator may be stopped. The repetition length of the scramblingsequence may be longer than the first length. In an example embodiment,the repetition length may be 32767 bits, and the first length may be 384bits. In an alternative example embodiment, the repetition length may be65535 bits, and the first length may be 192 bits.

For a second group of signaling bits, the PRBS generator of scramblerblock 710 is started again. However, it may not be initialized. Thus,the last value stored in the PRBS generator will be the start value ofthe next scrambling sequence. The second scrambling sequence for thesecond bit sequence may therefore be a continuation of the firstscrambling sequence. Likewise, a third and fourth scrambling sequencemay be generated by stopping and re-starting the PRBS generator withoutinitialization. The third and fourth scrambling sequences may be used toscramble a third and a fourth bit sequence corresponding to a third andfourth group of signaling bits.

In another example embodiment, the PRBS generator of scrambler block 710is initialized again for generation of a second P1 symbol. However, theinitialization value may be different than the initialization value forthe first scrambling sequence. In an example embodiment, aninitialization value “010010000001111” may be used. Further differentinitialization values may be used for the generation of a third andfourth P1 symbol.

In a further example embodiment, different PRBS generators may be usedfor generating a first and a second scrambling sequence.

In an example embodiment, the PRBS is read from a memory. Thus, a PRBSgenerator may not be needed. Different sequences may be taken from thePRBS. For example a first sequence may be taken from the PRBS for afirst scrambling sequence, a second sequence from the PRBS may be usedas a second scrambling sequence, and so on. Sequences taken from thePRBS may be overlapping. However, non-overlapping PRBS sequences mayprovide better correlation and/or synchronization properties.

FIG. 8 shows an example embodiment of an apparatus 800 configured totransmit frames or packets of a data stream, for example transmitter 104receiving a media stream at port 801, for example from service provider102 of FIG. 1. Coder 804 codes the media stream and producestransmission packets. P1 signaling information is generated at symbolgenerator 802. Symbol generator 802 may comprise the functional blocksof P1 symbol generator 700 as described in relation to FIG. 7. Signalinginformation may be provided through a bus system fromcontroller/processor 812. Signaling information may be stored in memory814, for example in a data area 816 of memory 814. More signalinginformation may be added to the transmission packets at packetizer 806.At least a first and a second P1 symbol are added to the transmissionpacket at packet merger 808. The completed packets are modulated in theassigned frequency band at modulator 810 and put out for transmission,for example to an antenna.

Controller/processor 812 controls the operation of the coder 804,packetizer 806, packet merger 808 and modulator 810. For example,controller/controller 812 defines the signaling parameters that areincluded in a first and a second group of signaling information.Controller/processor 812 may also define an initialization value forscrambler block 710 of P1 symbol generator 802. Controller may furtherdefine properties of the transmitted data, such as the bit rate, audiobandwidth, number of audio channels, audio codecs, video resolution,video frame rate, video codecs, and/or the like.

Apparatus 800 may further comprise memory 814 storing software forrunning apparatus 800. For example, software instructions for runningthe controller/processor 812 may be stored in a code area 818 of memory814. Memory 814 may comprise volatile memory, for example random accessmemory (RAM), and non volatile memory, for example read only memory(ROM), FLASH memory, or the like. Memory 814 may comprise one or morememory components. Memory 814 may also be embedded withcontroller/processor 812. Software comprising data and instructions torun apparatus 800 may also be loaded into memory 814 from an externalsource. For example, software may be stored on an external memory like amemory stick comprising one or more FLASH memory components, a compactdisc (CD), a digital versatile disc (DVD) 830, and/or the like. Softwareor software components for running apparatus 800 may also be loaded froma remote server, for example through the internet.

FIG. 9 is a flow diagram of a method 900 for transmitting P1 symbols assynchronization symbols according to an example embodiment of theinvention. At step 902, a first bit sequence is assigned to a firstgroup of signaling bits, for example a group comprising S1 and S2signaling bits as described in relation to TABLES 1 to 3. At step 904, asecond bit sequence is assigned to a second group of signaling bits. Thesecond group of signaling bits may be a copy of the first group ofsignaling bits. In an alternative embodiment, the second group ofsignaling bits comprises signaling information different from theinformation carried in the first group of signaling bits.

At step 906, the first bit sequence is scrambled with a first scramblingsequence, and at step 908 the second bit sequence is scrambled with asecond scrambling sequence different from the first scrambling sequence.The first and second scrambling sequences may be different parts of alonger random sequence, for example a PRBS. The parts of the longerrandom sequence may be overlapping. The parts may be parts of differentlonger random sequences.

At step 910, a first and a second OFDM symbol are assigned to the firstand second scrambled bit sequence. At step 912, the first and secondOFDM symbol are transmitted as synchronization symbols, for examplesynchronization symbols of a frame or a packet.

No special order is required for the steps the method of FIG. 9 relatedto the first group of signaling bits and for the blocks related to thesecond group of signaling bits. For example, the order may be changed sothat first the first group of signaling bits is handled. Thus, a firstbit sequence is assigned to the first group of signaling bits. Then, thefirst bit sequence is scrambled, and a first OFDM symbol is assigned tothe first scrambled bit sequence. After that, handling of the secondgroup of signaling bits is started.

Processing of the first bit sequence and the second bit sequence mayalso be performed in parallel. In an example embodiment, a first bitsequence is assigned to a first group of signaling bits. While the firstbit sequence is scrambled with a first scrambling sequence, a second bitsequence is assigned to a group of signaling bits. While the first OFDMsymbol is assigned to the first scrambled bit sequence, the second bitsequence is scrambled. While the first OFDM symbol is transmitted, asecond OFDM symbol is assigned to the second scrambled bit sequence.Parallel execution may continue in the same or in a similar way for athird and a fourth group of signaling bits. In an example embodiment,parallel execution may be performed by the P1 symbol generator 700 ofFIG. 7, as data is shifted through the blocks of the P1 symbolgenerator.

FIG. 10 is a flow diagram of a method 1000 for receiving a frame orpacket comprising P1 symbols as synchronization symbols according to anexample embodiment of the invention. At step 1002 an OFDM signal isreceived which comprises a first and a second OFDM symbol. The first andsecond OFDM symbol are used for synchronization of a data frame carriedin the OFDM signal at step 1004. The bit sequences of the first andsecond OFDM symbols are descrambled using a first and second scramblingsequence at steps 1006 and 1008, respectively. The second scramblingsequence is different from the first scrambling sequence. The scramblingsequences at the receiver correspond to the scrambling sequences at thetransmitter described in relation to FIG. 9. In an example embodiment,the scrambling sequences at the receiver are the same scramblingsequences as at the transmitter.

At step 1010, a first group of signaling bits is decoded from the firstdescrambled bit sequence. At step 1012, a second group of signaling bitsis decoded from the second descrambled bit sequence. In an exampleembodiment, the second group of signaling bits may carry the samesignaling information as the first group of signaling bits. In anexample embodiment, the information carried in the second group ofsignaling bits is different from the information of the first groupsignaling bits.

In an example embodiment, a third and/or a fourth OFDM symbol arereceived and used for synchronization. The bit sequences of the thirdand fourth OFDM symbol are descrambled using a third and fourthscrambling sequence. The third and fourth scrambling sequences aredifferent from the first and second scrambling sequence. In an exampleembodiment, the first to fourth scrambling sequences are continuousparts of a random sequence, for example of a PRBS generated by a PRBSgenerator at the receiver or stored in memory at the receiver. A thirdand fourth group of signaling bits are decoded from the third and fourthdescrambled bit sequences. In an example embodiment, the informationcarried in the third and fourth groups of signaling bits is differentfrom the information of the first and second groups of signaling bits.

FIG. 11 shows an example embodiment of a timing synchronization unit1100 for a receiver for the synchronization of up to four OFDM symbols.The up to four OFDM symbols may have a structure as described inrelation to FIG. 3. A base band signal is received at an input 1102 andfrequency shifted at 1104 with a frequency f_(SH), which denotes thefrequency shift between parts B 314/C 316 and part A 302 of the P1symbol 202. In the upper branch, the frequency shifted signal is delayedby a time T_(B) at T_(B) delay block 1106, conjugated, and multiplied bythe input signal. Thus, a correlation of parts B 314 and A1 304 may bedetected, for example after a running average filter 1114. In the lowerbranch, the input signal is delayed by a time T_(C) at T_(C) delay block1110 and multiplied by the frequency shifted and conjugated inputsignal. Thus, a correlation of parts A2 306 and C 316 may be detectedafter a running average filter 1116. In an example embodiment, thecorrelation between parts B and A1 may occur a time T_(A) before thecorrelation of A2 and C. Therefore, the signal from multiplier 1108 andfilter 1114 is delayed at T_(A) delay block 1118 by T_(A). In an exampleembodiment, the signal from delay block 1118 and filter 1116 ismultiplied at a multiplier 1120.

For an undisturbed signal, multiplier 1120 may output a signal peak forevery P1 symbol. As P1 symbols are transmitted in direct succession, asignal peak may occur for every P1 symbol at intervals T_(BAC), the timeof the P1 symbol 202 of FIGS. 2 and 3. Thus, the signal from multiplier1120 is delayed at T_(BAC) delay block 1122 for T_(BAC), at 2T_(BAC)delay block 1124 for 2*T_(BAC) and at 3T_(BAC) delay block 1126 for3*T_(BAC). The undelayed signal and the delayed signals are summed atsummation block 1128. A maximum peak of output 1130 may serve as asynchronization signal for the timing synchronization of a data frame orpacket, for example of data frame 542 of FIG. 5 or data frame 642 ofFIG. 6.

In an example embodiment, the signal from multiplier 1120 may be dividedinto multiple paths by using a switch that changes its position aftereach T_(BAC). The switch lets the signal from multiplier 1120 pass toone path at a time, for example to either a path containing one of theT_(BAC) delay block 1122, 2T_(BAC) delay block 1124, 3T_(BAC) delayblock 1126 or the path to summation block 1128. The separated signalsare delayed at the three delay blocks 1122, 1124, and 1126 for T_(BAC),2*T_(BAC) and 3*T_(BAC), respectively, and summed at summation block1128 as described above. By using a switch to select only one path at atime for the signal, multiple peaks or side peaks of output signal 1130may be avoided.

Synchronization unit 1100 may also work for less than four P1 symbols.However, the synchronization signal at output 1130 may have a lower peakas for four P1 symbols.

Summing the correlation peaks of up to four P1 symbols at summationblock 1128 may increase the level of the correlation peak relative to anoise level of the signal, as four P1 symbols may carry 4 times theenergy of a single P1 symbol. A detection threshold may be selectedaccordingly. Thus, synchronization of more than one P1 symbol may bemore robust or reliable. As the summation process at block 1128 is alinear process, a fractional frequency offset may still be detected fromoutput signal 1130 for frequency synchronization. The accuracy of afractional frequency offset detection may be increased, as the outputsignal is an average of four measurements with respect to the fractionalfrequency offset detection.

In an example embodiment, the number of delay blocks 1122, 1124, 1126may be adopted to the number of P1 symbols. For every additional P1symbol, one delay block is needed. Thus, for 2 P1 symbols, delay block1122 is needed, while delay blocks 1124 and 1126 may be removed from thesynchronization unit. For 3 P1 symbols, delay block 1122 and 1124 areneeded, while delay block 1126 may be removed. For 5 P1 symbols, onemore delay block with a delay of 4*T_(BAC) may be added.

FIG. 12 shows an example embodiment of an apparatus 1200 configured toreceive a broadcast transmission, for example apparatus 114, 116 ofFIG. 1. Apparatus 1200 may be a mobile apparatus, for example a mobilephone, or a stationary apparatus, for example a television set or amultimedia center. Apparatus 1200 comprises a receiver 1202 configuredto receive a transmission of digital multimedia transmission comprisinga media stream, for example a transmission according to the DVB-T2system or the DVB-NGH system. In an example embodiment, the transmissionmay be received through antenna 1228. In another example embodiment, thetransmission may be received through a cable connection. Receiver 1202may comprise a synchronization unit 1100 as described in relation toFIG. 11. An incoming frame or packet of the media stream is forwarded toa controller or processor 1204. Processor 1204 may be a digital signalprocessor (DSP), a microcontroller unit (MCU), a reduced instruction setcontroller (RISC), or any other kind of processor with sufficientprocessing capabilities. Processor 1204 may perform a packetdecapsulation and extraction of signaling information. Thus, processor1204 may extract lower layer signaling information comprised in multipleP1 symbols of the data frame or packet, and/or higher layer signalinginformation, for example signaling information comprised in at least oneP2 symbol, in an electronic service guide (ESG) and/or a sessiondescription file. Processor 1204 may further receive a synchronizationsignal from the synchronization unit of receiver 1202. Based at least inpart on the synchronization signal, processor 1204 may determine timinginformation for receiving the data frame or packet. Based at least inpart on the signaling information comprised in the multiple P1 symbols,processor 1204 may determine whether to receive the complete data frameor packet, or whether to stop reception of the data frame or packet.

Apparatus 1200 may comprise one or more memory blocks 1220. Memory 1220may comprise volatile memory 1222, for example random access memory(RAM). Volatile memory 1222 may be used to store data received fromreceiver 1202, for example data of a media stream at various processingand filtering stages, signaling data from the incoming frame or packet,configuration data for apparatus 1200, and/or the like. Processor 1204may communicate with memory blocks 1220 through a bidirectional bus 1206in order to read and store data and/or instructions.

Audio data from the data stream may be put out from processor 1204 toaudio decoder 1208. Audio decoder 1208 decodes the audio data andconverts the data to an analog audio signal. Analog audio signal may beplayed back on loudspeaker 1210. In an example embodiment, the analogaudio signal is played back on an audio headset.

Video data may be forwarded from processor 1204 to video decoder 1212which prepares the video data for play back on a display 1216 of userinterface 1214. User interface 1214 may further comprise a keyboard 1218for entering user data. User data may comprise a user preference, forexample a user preference for viewing a media stream at a certain videoand/or audio quality, resolution, frame rate, and the like. A userpreference may be used by processor 1204 to determine which audio andvideo data to receive and/or decode from the media stream.

Memory 1220 may also comprise non volatile memory 1224, for example readonly memory (ROM), FLASH memory, or the like. Non-volatile memory 1224may be used to store software instructions for processor 1204, forexample instructions to perform the process 1000 of FIG. 10. At least apart of memory 1220 may also be embedded with processor 1204. Softwarecomprising data and instructions to run apparatus 1200 may also beloaded into memory 1220 from an external source. For example, softwaremay be stored on an external memory like a memory stick comprising oneor more FLASH memory components, a compact disc (CD), a digitalversatile disc (DVD) 1230, or the like. Software or software componentsfor running apparatus 1200 may also be loaded from a remote server, forexample through the internet.

Without in any way limiting the scope, interpretation, or application ofthe claims appearing below, a technical effect of one or more of theexample embodiments disclosed herein is to make synchronization of aframe or packet more robust or reliable. Another technical effect of oneor more of the example embodiments disclosed herein is a higherrobustness against multipath propagation in single frequency networksdue to the different scrambling sequences. Another technical effect ofone or more of the example embodiments disclosed herein is a highersignaling capacity of the signaling in the multiple P1 symbols.

Embodiments of the present invention may be implemented in software,hardware, application logic or a combination of software, hardware andapplication logic. The software, application logic and/or hardware mayreside on an apparatus or an accessory to the apparatus. For example,the receiver may reside on a mobile TV accessory connected to a mobilephone. If desired, part of the software, application logic and/orhardware may reside on an apparatus, part of the software, applicationlogic and/or hardware may reside on an accessory. In an exampleembodiment, the application logic, software or an instruction set ismaintained on any one of various conventional computer-readable media.In the context of this document, a “computer-readable medium” may be anymedia or means that can contain, store, communicate, propagate ortransport the instructions for use by or in connection with aninstruction execution system, apparatus, or device, such as a computer,with one example of a computer described and depicted in FIG. 8 andanother example of a computer described and depicted in FIG. 12. Acomputer-readable medium may comprise a computer-readable storage mediumthat may be any media or means that can contain or store theinstructions for use by or in connection with an instruction executionsystem, apparatus, or device, such as a computer.

If desired, the different functions discussed herein may be performed ina different order and/or concurrently with each other. Furthermore, ifdesired, one or more of the above-described functions may be optional ormay be combined.

Although various aspects of the invention are set out in the independentclaims, other aspects of the invention comprise other combinations offeatures from the described embodiments and/or the dependent claims withthe features of the independent claims, and not solely the combinationsexplicitly set out in the claims.

It is also noted herein that while the above describes exampleembodiments of the invention, these descriptions should not be viewed ina limiting sense. Rather, there are several variations and modificationswhich may be made without departing from the scope of the presentinvention as defined in the appended claims.

What is claimed is:
 1. A method comprising: assigning a first bitsequence of a first length to a first group of signaling bitsinformation; generating a first scrambling sequence and scrambling thefirst bit sequence with the first scrambling sequence; assigning a firstorthogonal frequency division multiplexing symbol to the first scrambledbit sequence scrambled by the first scrambling sequence; andtransmitting the first orthogonal frequency division multiplexing symbolas a synchronization symbol of a data frame, wherein the first group ofsignaling bits orthogonal frequency division multiplexing symbolindicates whether a next symbol comprises a second orthogonal frequencydivision multiplexing symbol as an additional synchronization symbol,wherein the second orthogonal frequency division multiplexing symbolcomprises is assigned to a second bit sequence scrambled by a secondscrambling sequence and, wherein the second sequence is assigned to asecond group of the signaling bits information, wherein the first andthe second bit sequences have the same length, and wherein the firstscrambling sequence is generated by a pseudo random binary sequencegenerator using a first initialization value, and wherein and the secondscrambling sequence is generated by the are different parts of apseudorandom binary sequence generator using a second initializationvalue having a repetition length greater than each length of the firstand the second sequences.
 2. The method according to of claim 1, whereinthe second group of signaling bits sequence comprises informationdifferent from the first group of signaling bits sequence.
 3. The methodaccording to of claim 1, wherein the data frame is a data frame of adigital video broadcast.
 4. A method comprising: receiving an orthogonalfrequency division multiplexing signal comprising a first orthogonalfrequency division multiplexing symbol; synchronizing a data frame basedat least in part on the first orthogonal frequency division multiplexingsymbol, wherein the first orthogonal frequency division multiplexingsymbol corresponds to a first scrambled bit sequence; generating a firstsequence by descrambling the first scrambled bit sequence with a firstscrambling sequence; and decoding a first group of signaling bitsinformation from the first descrambled bit sequence, wherein the firstgroup of signaling bits orthogonal frequency division multiplexingsymbol indicates whether a next symbol comprises a second orthogonalfrequency division multiplexing symbol as an additional synchronizationsymbol; determining from the first group of signaling bits orthogonalfrequency division multiplexing symbol whether the second orthogonalfrequency division multiplexing synchronization symbol is present, andin response to determining that the second orthogonal frequency divisionmultiplexing synchronization symbol is present: receiving the secondorthogonal frequency division multiplexing synchronization symbolcorresponding to a second scrambled bit sequence; generating a secondsequence by descrambling the second scrambled bit sequence with a secondscrambling sequence; and decoding a second group of the signaling bitsinformation from the second descrambled bit sequence, wherein the firstand the second bit sequences have the same length, and wherein the firstscrambling sequence is generated by a pseudorandom binary sequencegenerator using a first initialization value, and wherein and the secondscrambling sequence is generated by the are different parts of apseudorandom binary sequence generator using a second initializationvalue having a repetition length greater than each length of the firstand the second sequences.
 5. The method according to of claim 4, whereinthe second group of signaling bits sequence comprises informationdifferent from the first group of signaling bits sequence.
 6. Anapparatus, comprising: at least one hardware processor; and at least onememory including computer program code, the at least one memory and thecomputer program code configured to, with the at least one hardwareprocessor, cause the apparatus to perform at least the following: assigna first bit sequence of a first length to a first group of signalingbits information; generate a first scrambling sequence and scramble thefirst bit sequence with the first scrambling sequence; assign a firstorthogonal frequency division multiplexing symbol to the first scrambledbit sequence scrambled by the first scrambling sequence; and transmitthe first orthogonal frequency division multiplexing symbol as asynchronization symbol of a data frame, wherein the first group ofsignaling bits orthogonal frequency division multiplexing symbolindicates whether a next symbol comprises a second orthogonal frequencydivision multiplexing symbol as an additional synchronization symbol,wherein the second orthogonal frequency division multiplexing symbolcomprises is assigned to a second bit sequence scrambled by a secondscrambling sequence and, wherein the second sequence is assigned to asecond group of the signaling bits information, wherein the first andthe second bit sequences have the same length, and wherein the firstscrambling sequence is generated by a pseudo random binary sequencegenerator using a first initialization value, and wherein and the secondscrambling sequence is generated by the are different parts of apseudorandom binary sequence generator using a second initializationvalue having a repetition length greater than each length of the firstand the second sequences.
 7. A non-transitory computer program productcomprising a computer-readable medium bearing computer program codeembodied therein for use with encoded with instructions that whenexecuted by a computer, the computer program code comprising causes anapparatus to: code for assigningassign a first bit sequence of a firstlength to a first group of signaling bits; code for generatinggenerate afirst scrambling sequence and code for scramblingscramble the first bitsequence with the first scrambling sequence; code for assigningassign afirst orthogonal frequency division multiplexing symbol to the firstscrambled bit sequence; and code for transmittingtransmit the firstorthogonal frequency division multiplexing symbolssymbol as asynchronization symbol of a data frame, wherein the first group ofsignaling bits indicates whether a next symbol comprises a secondorthogonal frequency division multiplexing symbol as an additionalsynchronization symbol, wherein the second orthogonal frequency divisionmultiplexing symbol comprises a second bit sequence scrambled by asecond scrambling sequence and assigned to a second group of signalingbits, wherein the first and second bit sequences have the same length,wherein the first scrambling sequence is generated by a pseudorandompseudorandom binary sequence generator using a firstinitialization value, and wherein the second scrambling sequence isgenerated by the pseudorandom binary sequence generator using a secondinitialization value.
 8. A non-transitory computer-readable mediumencoded with instructions that, when executed by a computer, performcause an apparatus to: assigningassign a first bit sequence of a firstlength to a first group of signaling bits information;generatinggenerate a first scrambling sequence and scramblingscramblethe first bit sequence with the first scrambling sequence;assigningassign a first orthogonal frequency division multiplexingsymbol to the first scrambled bit sequence scrambled by the firstscrambling sequence; and transmittingtransmit the first and secondorthogonal frequency-division frequency division multiplexing symbolssymbol as a synchronization symbols symbol of a data frame, wherein thefirst group of signaling bits orthogonal frequency division multiplexingsymbol indicates whether a next symbol comprises a second orthogonalfrequency division multiplexing symbol as an additional synchronizationsymbol, wherein the second orthogonal frequency division multiplexingsymbol comprises a second bit sequence scrambled by a second scramblingsequence and, wherein the second sequence is assigned to a second groupof the signaling bits information, wherein the first and the second bitsequences have the same length, and wherein the first scramblingsequence is generated by a pseudo random binary sequence generator usinga first initialization value, and wherein and the second scramblingsequence is generated by the are different parts of a pseudorandombinary sequence generator using a second initialization value having arepetition length greater than each length of the first and the secondsequences.
 9. An apparatus, comprising: at least one hardware processor;and at least one memory including computer program code, the at leastone memory and the computer program code configured to, with the atleast one hardware processor, cause the apparatus to perform at leastthe following: receive an orthogonal frequency-division multiplexingsignal comprising a first orthogonal frequency-division frequencydivision multiplexing symbol; synchronize a data frame based at least inpart on the first orthogonal frequency division multiplexing symbol,wherein the first orthogonal frequency division multiplexing symbolcorresponds to a first scrambled bit sequence; descramblegenerate afirst sequence by descrambling the first scrambled bit sequence with afirst scrambling sequence; and decode a first group of signaling bitsinformation from the first descrambled bit sequence, wherein the firstgroup of signaling bits orthogonal frequency division multiplexingsymbol indicates whether a next symbol comprises a second orthogonalfrequency division multiplexing symbol as an additional synchronizationsymbol; determine from the first group of signaling bits orthogonalfrequency division multiplexing symbol whether the second orthogonalfrequency division multiplexing synchronization symbol is present, andin response to determining a determination that the second orthogonalfrequency division multiplexing synchronization symbol is present:receive the second orthogonal frequency division multiplexingsynchronization symbol corresponding to a second scrambled bit sequence;descramblegenerate a second sequence by descrambling the secondscrambled bit sequence with a second scrambling sequence; and decode asecond group of the signaling bits information from the seconddescrambled bit sequence, wherein the first and the second bit sequenceshave the same length, and wherein the first scrambling sequence isgenerated by a pseudorandom binary sequence generator using a firstinitialization value, and wherein and the second scrambling sequence isgenerated by the are different parts of a pseudorandom binary sequencegenerator using a second initialization value having a repetition lengthgreater than each length of the first and the second sequences.
 10. Acomputer program product comprising a non-transitory computer-readablemedium bearing computer program code embodied therein for use with acomputer, the computer program code comprising encoded with instructionsthat when executed by a computer, cause an apparatus to: code forreceivingreceive an orthogonal frequency division multiplexing signalcomprising a first orthogonal frequency division multiplexing symbol;code for synchronizingsynchronize a data frame based at least in part onthe first orthogonal frequency division multiplexing symbolssymbol,wherein the first orthogonal frequency division multiplexing symbolcorresponds to a first scrambled bit sequence; code forgenerate a firstsequence by descrambling the first scrambled bit sequence with a firstscrambling sequence; and code for decoding a first group ofdecodesignaling bitsinformation from the first descrambled bit sequence and,wherein the first group of signaling bits orthogonal frequency divisionmultiplexing symbol indicates whether a next symbol comprises a secondorthogonal frequency division multiplexing symbol as an additionalsynchronization symbol; code for determiningdetermine from the firstgroup of signaling bitsorthogonal frequency division multiplexing symbolwhether the second orthogonal frequency division multiplexingsynchronization symbol is present, and in response to determining adetermination that the second orthogonal frequency division multiplexingsynchronization symbol is present: code for receivingreceive the secondorthogonal frequency division multiplexing synchronization symbolcorresponding to a second scrambled bit sequence; code forgenerate asecond sequence by descrambling the second scrambled bit sequence with asecond scrambling sequence; and code for decoding a second groupofdecode the signaling bitsinformation from the second descrambled bitsequence, wherein the first and the second bit sequences have the samelength, and wherein the first scrambling sequence is generated by apseudorandom binary sequence generator using a first initializationvalue, and wherein and the second scrambling sequence is generated bythe are different parts of a pseudorandom binary sequence generatorusing a second initialization value having a repetition length greaterthan each length of the first and the second sequences.
 11. The methodof claim 1, wherein the first scrambling sequence is generated by apseudorandom binary sequence generator using a first initializationvalue, and wherein the second scrambling sequence is generated by thepseudorandom binary sequence generator using a second initializationvalue.
 12. The method of claim 1, wherein the second sequence isidentical to the first sequence.
 13. The method of claim 4, wherein thefirst scrambling sequence is generated by a pseudorandom binary sequencegenerator using a first initialization value, and wherein the secondscrambling sequence is generated by the pseudorandom binary sequencegenerator using a second initialization value.
 14. The method of claim4, wherein the second sequence is identical to the first sequence. 15.The method to claim 4, wherein the data frame is a data frame of adigital video broadcast.
 16. The apparatus of claim 6, wherein the firstscrambling sequence is generated by a pseudorandom binary sequencegenerator using a first initialization value, and wherein the secondscrambling sequence is generated by the pseudorandom binary sequencegenerator using a second initialization value.
 17. The apparatus ofclaim 6, wherein the second sequence is identical to the first sequence.18. The apparatus to claim 6, wherein the data frame is a data frame ofa digital video broadcast.
 19. The apparatus of claim 9, wherein thefirst scrambling sequence is generated by a pseudorandom binary sequencegenerator using a first initialization value, and wherein the secondscrambling sequence is generated by the pseudorandom binary sequencegenerator using a second initialization value.
 20. The apparatus ofclaim 9, wherein the second sequence is identical to the first sequence.21. The apparatus to claim 9, wherein the data frame is a data frame ofa digital video broadcast.